(1) Field of the Invention
The present invention relates to a graphite fiber. More particularly, it relates to a graphite fiber, which is valuable for a composite material having a plastic as a matrix and having a high compression strength.
(2) Description of the Related Art
Since carbon fibers have a specific strength or specific elastic modulus higher than that of metallic materials, composite materials composed of a carbon fiber and a plastic matrix are now widely used in golf shafts, fishing rods and the like in the fields of sports and leisure and as light-weight structural materials mainly for aircraft, spaceship and communication satellite. With the increased demand for these composite materials, a further improvement of the quality is desired, and great advances have been made in the improvement of the tensile strength. However, the compression strength has not been similarly improved, and a problem of an unbalance between the compression strength and tensile strength has arisen.
In graphite fibers having an especially high elastic modulus, a reduction of the weight is generally realized by effectively utilizing the rigidity to reduce the thickness of a structural material. However, in this case, the attainment of the intended weight-reducing effect is restricted by the resulting poor compression strength.
As is well-known, a carbon fiber is obtained by calcining an organic fiber of cellulose, polyacrylonitrile or pitch at a high temperature in an inert gas. In general, the final calcination temperature is higher than 1,000.degree. C., and especially in the case of a graphite fiber, the final calcination temperature sometimes exceeds 2,000.degree. C.
Where polyacrylonitrile is used as the starting material, it is widely known that, to obtain a carbon fiber having a high strength and high elastic modulus, it is an important requirement that, at the step of preparing the starting fiber, a high draw ratio be adopted to produce a highly oriented structure and the highly oriented fiber be calcined under tension. Carbon can take two crystal structures, i.e., a diamond structure and a graphite structure. In general, the carbon fiber has a graphite structure comprising a laminated net planes. This graphite structure has a much higher anisotropy than an ordinary crystal structure of a metal, and the mechanical characteristics in the direction of the fiber axis are enhanced by orienting the net planes selectively in the direction of the fiber axis.
To realize a high tensile strength, an enhancement of the completeness of the crystal structure as mentioned above, and a prevention of bonding among filaments, and a removal of surface defects such as foreign substances, impurities and mechanical damage are important, and many techniques for improving the strength based on this understanding have been proposed.
There have been little investigations into or proposals for an improvement of the compression strength in carbon fiber-reinforced composite materials. Only Japanese Unexamined Patent Publication No. 59-118,203 teaches that, if the single fiber thickness is increased, the compression strength of the composite material is improved. Indeed, it is considered that, in a fiber-reinforced composite material, the size of the constituent fiber probably has an influence on the compression strength. However, in the case of a brittle material such as a carbon fiber, an increase of the fiber diameter results in an increase of the probability of an inclusion of defects, and an attainment of a high strength becomes difficult. Furthermore, since carbonization of a carbon fiber is carried out by a thermal decomposition reaction in the solid phase, a long reaction time is needed for a uniform graphitization in the case of a carbon fiber having a large diameter. Accordingly, the process becomes economically disadvantageous.
Therefore, the development of a matrix having a higher rigidity attracted more attention than the search for a solution in the carbon fiber per se. In other words, few trials have been made into improvements of the compression strength of a carbon fiber-reinforced composite material by improving the carbon fiber.
The object of the present invention is to provide a carbon fiber valuable as a composite material having a high compression strength by rationalizing the inner structure of the carbon fiber, contrary to the conventional technique. The objective fiber of the present invention is a graphite fiber having an elastic modulus of at least 340 GPa. This is because in the case of, for example, a polyacrylonitrile carbon fiber, as the elastic modulus is increased, the compression strength of the composite material is drastically reduced. Furthermore, as pointed out hereinbefore, although it is considered that the characteristics of a carbon fiber having a high elastic modulus will enable practical use thereof as a thin structural material, expansion of this use is often obstructed by the poor compression strength.